The present invention relates to watthour meters using isolation transformers for sensing the line voltage across a load.
A typical watthour meter includes means for sensing the load current and line voltage drawn by a load from an AC power source. The current and voltage signals thus derived are multiplied to produce a signal corresponding to the amount of AC electrical energy consumed by the load. This multiplication function is frequently performed by a solid state circuit to replace the mechanical metering methods previously used.
In electronic watthour meters of the type described, the line voltage is typically sensed using a voltage transformer connected across the AC source. The primary and secondary coils of these voltage transformers are generally heavily insulated to account for high voltage transients across the primary coil induced, for instance, by a lightning strike between the AC source and the watthour meter. Particularly in electronic watthour meters, electric isolation between the primary and secondary coils is important to protect the solid state components of the meter as well as to protect the customer or meter-reader from electrical shock. Because of the amount of isolation required to achieve proper isolation, voltage transformers are typically large and expensive.
As watthour meters become more sophisticated, employing more solid-state circuitry, there is an emphasis on reducing the size and expense of the meter components, including the voltage sensing means. Thus, it is an object of the present invention to provide a replacement for the typical voltage transformer that is smaller and less expensive. Another object is to provide voltage sensing means in a watthour meter that maintains the acceptable electrical isolation characteristic required for watthour meters.